1. Field of the Invention
The present invention relates to multiplexing two signals for simultaneous transmission over telephone lines and other communication paths.
2. Description of the Prior Art
As the cost of installing communication lines increases, it has become desirable to simultaneously send more than one signal over a single communication line between a transmitter and a receiver. Prior art techniques adapted for simultaneous transmission of two signals, such as one of digital data and one analog signal, are frequency division multiplexing and time division multiplexing.
However, in frequency division multiplexing, the digital data was typically in phase shift keyed (PSK) format. Circuitry was required for the PSK signal to be translated to higher frequencies outside the frequency spectrum of the analog signal in order that frequency selective filtering at the receiver could separate the two signals from each other. This type of frequency division multiplexing required either a very wide signal transmission bandwidth or, alternatively, complex frequency translation equipment at the transmitter and highly selective filters at the receiver.
In time division multiplexing the digital data was encoded in a suitable format. The analog signal was then sampled at a required sampling rate, quantized and encoded into a suitable digital code and then multiplexed, or transmitted in interleaved sequence, with the already encoded digital data. Time multiplexing thus required complex sampling and encoding circuitry at the transmitter, a very wide transmission bandwidth and similarly complex sampling and decoding equipment.